Cemented carbides are technologically important materials which offer unique combinations of properties such as strength, hardness, toughness, and wear resistance. They are extensively employed in metal forming tools and metal cutting tools. For example, important industrial applications of cemented carbides include rings for rolling metals as well as knives for slitting and side trimming of sheet and strip metals.
In general, cemented carbides comprise one or more "hard" phases (usually carbides of the elements belonging to groups IVB through VIB of the periodic table, e.g., W, Ti, Ta, Nb, Cr, V, Mo, Hf and Zr) cemented or bound together by a softer binder phase (usually metals belonging to the Fe-group, i.e., Co, Ni, or Fe, or alloys of these metals). Cemented carbides are prepared using powder metallurgy techniques. The principal steps in such a fabrication process include preparing an intimate blend of carbide and binder metal particles, compressing the blended particles (powder) to the desired shape in a rigid or flexible die, and liquid-phase sintering of the resultant powder compact to achieve the fully dense cemented carbide article. The composition and fabrication of cemented carbide articles are discussed in detail in Schwarzkopf, P. and Kieffer, R., Cemented Carbides, The MacMillan Company, New York (196), the disclosure of which is incorporated herein by reference.
The properties of cemented carbides are determined by the overall composition, including the identity of the carbide, the identity of the binder and the fraction of binder present (i.e., the ratio of binder to carbide), and the average grain size of the hard phase. A metallurgist may vary one or more of these parameters to design cemented carbides for different applications.
In the general case, as grain size increases (i.e., increasing coarseness) the hardness of the article decreases and the toughness increases. Increasing the binder fraction has an effect similar to increasing grain size (i.e., the hardness decreases and the toughness increases). The properties of a cemented carbide article are thus optimized by appropriately selecting the overall composition and grain size. The effect of the composition and grain size upon the properties of cemented carbides is discussed in detail in Schwarzkopf et al., supra.
Because of inherent limitations of the conventional powder metallurgy process described above, cemented carbide articles are invariably fabricated having a substantially uniform composition and microstructure (on a scale of millimeters or centimeters), and hence, substantially uniform properties, throughout the volume of the article. Many such substantially homogeneous compositions exist in the prior art.
However, it is desirable in a number of applications to fabricate cemented carbide articles having nonuniform properties. In that regard, the spatial composition of cemented carbide articles has, in the past, been altered in a very limited sense by varying the distribution of the concentration of the single metallic carbides throughout the article.
Recently, for example, a process for producing cemented carbides comprising a single carbide but having different compositions and/or microstructures at different locations within an article has been described in a report by Sandvik Hardmetals in Metals Powder Report (December 1992). The Sandvik technique (known as Dual-Phase or DP Technology) consists of carburizing cemented carbide articles which initially have lower than normal levels of carbon. During the carburizing process, a compositional gradient is created within the article, resulting in slightly different property levels at different locations within the article.
The Sandvik method is severely limited, however, in that it does not enable fabrication of materials with substantially different compositional variations. For example, combining a Co-based cemented carbide with a Ni-based cemented carbide or even having a single-element carbide in which the composition (carbide/binder ratio) varies widely is not possible under the Sandvik process. Furthermore, it is virtually impossible using the Sandvik process to fabricate articles with substantially different grain sizes in different regions within an article.
Nevertheless, in many applications it is desirable to have substantially different properties and/or compositions at different locations in the cemented carbide article. For example, it may be desirable to have high wear resistance near the surface of an article but a high level of toughness in the interior of the article. However, toughness invariably decreases if the material is designed to have high hardness and wear resistance. Such a "dual-property" material is impossible to fabricate in cemented carbide articles using conventional powder metallurgy techniques (as such articles invariably have a substantially homogeneous composition and microstructure throughout the volume of the article).
Furthermore, it may be desirable for the working volume of a cemented carbide article to comprise a high-grade (and expensive) cemented carbide with a certain level of properties while the remaining portion of the article comprises a less expensive, lower grade of material. This result is likewise impossible to achieve under conventional powder metallurgy processes.
It is, therefore, desirable to develop a method of producing novel cemented carbide articles comprising multiple grades of cemented carbides. Each portion of such cemented carbide articles could be manufactured to exhibit different properties by virtue of its distinct composition and/or microstructure (i.e., its distinct grade).